This invention relates to the field, now well established, of use of ansa-metallocenes as catalysts. They are particularly useful as catalysts for the polymerization of ethylene and alpha olefins, such as propylene.
Conventional heterogeneous catalysts, such as Ziegler-Natta systems, have a variety of active sites, only some of which are stereo-specific. Obtaining a polymer with specific properties can involve a considerable amount of trial and error in order to find the best combination of catalyst, co-catalyst and stereo-regulator. In contrast, however, the active polymerization site in a metallocene catalyst is well defined, and can be modified in a straightforward manner via modification by the cyclopentadienyl ligands, enabling the structure of the polymer to be controlled with far greater precision.
A simple metallocene catalyst for polymerizing ethylene is (C.sub.5 H.sub.5).sub.2 ZrCl.sub.2, which consists of a zirconium atom bound to two chlorine atoms and two cyclopentadienyl rings, and which is activated by cocatalysts such as methylaluminoxane (MAO). During the 1980's, ansa or bridged metallocenes, in which the cyclopentadienyl rings are linked by a chemical bridge, were found to be particularly useful for the polymerization of olefins. In particular, ansa-metallocene complexes, when used in combination with a cocatalyst such as methylaluminoxane (MAO), polymerize propylene to highly isotactic polypropylene, highly syndiotactic polypropylene, or atactic polypropylene, depending on the structure of the ansa-metallocene used.
As is well known, isotactic polymers have each pendant group attached to the backbone in the same orientation, whereas in syndiotactic polymers, these groups alternate in their orientations, and atactic polymers have a random arrangement of the groups along the backbone. Since the stereochemistry of the polymer has a great effect on its properties, it is desirable to control this feature. Chiral, C.sub.2 -symmetric ansa-metallocenes produce isotactic polypropylene.
While the greatest area of potential use for ansa-metallocene catalysts currently is for polymerization of olefins, such as ethylene and propylene, they also have significant uses as catalysts or catalyst precursors for other reactions where stereo-selectivity is important.
The utility of ansa-metallocene complexes as catalysts for olefin polymerization and other reactions has created a high demand for a practical synthesis of ansa-metallocene compounds.
In spite of this demand, current procedures for the synthesis of Group 4 (Ti,Zr,Hf) ansa-metallocenes require the use of ansa-bis-cyclopentadienyl dianion reagents and are hampered by low yields and tedious isomer separation and purification steps. Some of these problems have been discussed in Ellis, W. W.; Hollis, T. K.; Odenkirk, W., Whelan, J.; Ostrander, R.; Rheingold, A. L.; Bosnich, B. Organometallics 1993, 12, 4391. In particular, the synthesis of chiral C.sub.2 symmetric ansa-metallocenes typically produces mixtures of desired rac (racemic) and undesired meso isomers. A typical synthesis of an ansa-metallocene complex is shown in Equation 1 below: ##STR1##
This equation is typical of the process as shown in the art. See for example Spaleck, W.; Kuber, F., Winter, A.; Rohrman, J.; Bachmann, B.; Antberg, M.; Dolle, V.; Paulus, E. F. Organometallics 1994, 13, 954. Stehling, U.; Diebold, J.; Kirsten, R.; Roll, W.; Brintzinger, H. H.; Jungling, S.; Mulhaupt, R.; Langhauser, F. Organometallics 1994, 13, 964. Halterman, R. L. Chem. Rev. 1992, 92, 965. See also, for example, U.S. Pat. No. 5,145,819, U.S. Pat. No. 5,268,495, and EPA 0-530-908-A1.
By way of further example, an important chiral Group 4 ansa-metallocene is rac-(EBI)ZrCl.sub.2 (EBI=ethylene-1,2-bis(1-indenyl) which is currently prepared from ZrCl.sub.4 and the dianion of the EBI ligand (Halterman, R. L. Chem. Rev. 1992, 92, 965). Brintzinger (Wild, F. R. W. P.; Wasiucionek, M.; Huttner, G., Brintzinger, H. H. J. Organomet. Chem. 1985, 288, 63) and Collins (Collins, S.; Kuntz, B. A.; Hong, Y. J. Org. Chem. 1989, 54, 4154; Collins, S.; Kuntz, B. A.; Taylor, N. J.; Ward, D. G. J. Organomet. Chem. 1988, 342, 21) used (EBI)Li.sub.2 and reported low, variable yields (20-50%) of rac-(EBI)ZrCl.sub.2. Buchwald employed (EBI)K.sub.2 and obtained (EBI)ZrCl.sub.2 in a rac/meso ratio of 2/1 in 70% yield. Grossman, R. B.; Doyle, R. A.; Buchwald, S. L. Organometallics 1991, 10, 1501. In general, current synthetic procedures produce the desired rac ansa-metallocenes in 10%-30% yield after tedious separation and purification steps, and even then separation of the rac from the meso products is not always possible.
Lappert et al. (Chandra, G.; Lappert, M. F. J. Chem Soc. (A) 1968, 1940) reported that certain Group 4 metallocene complexes are formed by the reaction of Group 4 metal amide complexes with cyclopentadiene compounds. However, this reaction yields only mono-cyclopentadienyl products when the metal is titanium, or when the cyclopentadiene compound is indene. This was ascribed to steric hindrance which disfavors addition of the second cyclopentadienyl ligand when the metal is small (titanium) or the cyclopentadienyl ligand is bulky (indenyl). Hefner. et al., also (U.S. Pat. No. 5,194,532) discusses the preparation of (indenyl)Ti(NMe.sub.2).sub.3 by reaction of indene and Ti(NMe.sub.2).sub.4. Ansa-metallocene complexes are not discussed in the Lappert or Hefner references.
An earlier invention of Richard F. Jordan, et al., entitled Synthesis of Ansa-Metallocene Catalysts, filed Jun. 1, 1994 and issued Feb. 27, 1996 as U.S. Pat. No. 5,495,035, and incorporated herein by reference, relates to a process of preparing rac ansa-metallocene amide complexes ((Cp--X--Cp)m(NRR').sub.n-2) in high yield by reacting an ansa-bis-cyclopentadiene, indene, fluorene, or substituted derivative thereof with a metal amide complex wherein the metal is a Group 4 metal, preferably zirconium, and R and R' (Eq. 2) are preferably hydrogen or C.sub.1 to C.sub.20 hydrocarbyl radicals, and more preferably C.sub.1 to C.sub.4 alkyl, and most preferably methyl. The resulting amide is converted to a chloride ((Cp--X--Cp)mCl.sub.n-2) before use as a catalyst. It would be more efficient if a process could be developed which directly used the amide as a catalyst without the need for the conversion to the chloride.
It is therefore an object of the present invention to provide a process for producing polyolefins, particularly isotactic polypropylene from rac ansa-metallocene metal amide complexes directly without the need to convert the amide to a chloride before use as a catalyst.
The method of accomplishing the above objective, as well as others, will become apparent from the detailed description of the invention which follows.